In the offshore production of oil and gas, equipment located under the water surface controls and directs the flow of oil, gas, and other production fluids from the wellbore to the water surface. Typically, the equipment is attached to a subsea base rigidly connected to the upper end of a well casing. Production tubing located within the well casing is connected to equipment such as a Christmas tree. The Christmas tree usually comprises control valves, pressure gauges, and chokes to monitor and to control the flow of the production fluids after the well has been drilled and completed. The production fluids are directed by a riser from the Christmas tree to a vessel or platform deck located at the water surface. The riser may be articulated with swivels to permit the riser to flex in response to loading forces induced by waves and ocean currents.
The valves, swivels, and other subsea components used in the production of oil and gas will eventually become worn and must be replaced. In shallow water, divers are used to perform such maintenance operations. At greater depths, the complexity and cost of manual maintenance operations increases. To simplify the replacement of undersea components at depths beyond the reach of conventional diving operations, the components of an underwater equipment package are often bundled in modular units which can be retrieved from a vessel located on the water surface. However, the concept of modular units is inefficient because single components cannot be replaced without retrieving the entire module. In addition, modular units are expensive to design and to fabricate due to the additional work necessary to ensure a proper connection between adjacent modules.
To avoid the inefficiencies associated with modular designed systems, remotely operated underwater vehicles are frequently used to replace defective underwater components and to perform other maintenance operations. Remotely operated vehicles are useful because they can be mobilized quickly and can be operated from the water surface. However, the size and weight of remotely operated vehicles limits the maneuverability of such vehicles in performing sophisticated underwater maneuvers.
Accordingly, a need exists for a method and apparatus which simplifies the alignment of undersea components. The apparatus should be easy to construct and the method should reduce the operating time necessary to align submerged components.